The present invention relates to methods and apparatus for measuring the gas flow volume in a system of the kind in which the amount of gas flowing in a duct can vary in response to variation of the area of a variable orifice in the duct and/or in response to changes in the total pressure of the gas upstream of the orifice.
This invention relates particularly to methods and apparatus for providing highly accurate measurements of gas flow at quite low gas flow volumes within the duct.
The present invention has particular application as a control for the air flow in a duct of a variable air volume air conditioning system of the kind used in buildings.
The present invention also has application to the measurement and to the control of gas flow for other gas flow volume systems. The present invention may, for example, be used to measure and to control the gas flow in an automotive system of the kind in which it is necessary to sense the level of a pollutant, e.g. the level of nitrogen oxide, in the exhaust gases of an internal combustion engine.
Obtaining an accurate measurement of quite low air flow volume (and exerting effective control over such low air flows) can present a number of practical problems in variable air volume air conditioning systems for buildings. The overall variable air volume system for an entire building is quite large and often requires a relatively large number of control devices for measuring and for controlling the air flow to localized zones or areas, such as individual rooms or suites within the building. Because a typical system may require a large number of control devices, it is desirable that the control devices be of a class which is not too expensive. The control devices should, however, be capable of producing accurate, repeatable flow measurements at both high flow velocities (in the range of 4,000 feet per minute in a typical six inch diameter duct) and at low flow velocities (in the range of 400 feet per minute or less). The devices should also be capable of being easily installed in the field and should require little or no field calibration or recalibration after installation.
Designing and manufacturing a control device which will meet all the requirements at high flow volumes is not too difficult.
But it is difficult to manufacture a control device which is cost effective and which performs satisfactorily for accurate measurement and control of flow volumes at low flow rates (in the range of 400 feet per minute or less).
Many of the existing measurement devices require an accurate sensing of the velocity pressure. The velocity pressure is a low number at low flow volumes. For example, in a conventional 6 inch diameter duct which produces one inch water column velocity pressure at 4,000 feet per minute flow velocity within the duct, a flow velocity of 400 feet per minute produces a velocity pressure of only 0.01 inch water column. If the accuracy of the measuring device is 1 percent at the one inch water column corresponding to 4,000 feet per minute (that is if the accuracy is plus or minus 0.01 inch water column at the high end) then the device can be off by factor of 100 percent (plus or minus 0.01 inch water column) at the 400 feet per minute flow velocity in the duct. There are many mechanical means to amplify, but such amplifier means are effective only at highest flow rates. The amplifiers present problems of measuring low flows.
It is a primary object of the present invention to overcome the problems associated with obtaining accurate measurements of gas flow volumes at quite low flow velocities.
It is a related object of the present invention to obtain and to use for gas flow measurement purposes, in a single damper system, a sensed pressure signal which provides a large amplitude signal (high number) at all flow volumes, including flow velocities which are quite small so that highly accurate flow measurements can be obtained even at the small flow velocities.
It is an object to achieve the foregoing by referencing the total pressure upstream of the damper to the static pressure downstream of the damper so that the sensed pressure condition is a condition which is representative of the differential pressure between the total pressure upstream of the damper and the static pressure downstream of the damper and is a signal which always has a high amplitude even at quite low gas flow velocities and volumes.
It is another, related object to do the pressure signal sensing in a way such that the basic indication of the air flow within the duct can be obtained by sensing the damper position and the basic indication of the air flow is not dependent upon obtaining an accurate indication of just a velocity pressure per se within the duct.
It is another object of the present invention to produce a manufactured product of consistent, predictable, reproducable geometry using standard sensors and which can be calibrated at the factory by locating one point on a defined database, identified by using one orifice area opening (one damper position) signal and one differential pressure signal generated at a known volume of air through the system.